A common type of compression system used in a motor vehicle air conditioner is the positive displacement compressor, which may be the fixed or variable displacement type. Although there are a variety of compressor configurations, such positive displacement compressors generally operate on the same basic principle: a certain inlet volume of gas is confined in a given space, compressed as the size of the space is reduced, and then expelled at the higher pressure.
One type of positive displacement compressor incorporates reciprocating pistons to impart compression. In one compressor design, a solid cylindrical piston moves axially within a cylindrical bore in the compressor housing. On the down stroke (or suction stroke), a low pressure area is created, causing gas in a suction line to pass through a suction valve into the compression chamber. On the up stroke (or compression stroke), the gas in the compression chamber is compressed as the area in which the gas is held (or clearance space) is reduced. At a certain pressure, the compressed gas is released through a discharge (or delivery) valve into a discharge line. Ultimately, this cycle results in the release of chilled air into the interior of the motor vehicle. The linear upward and downward motion along the axis of the bore is articulated to each piston through an attached connecting rod and/or piston rod, and associated crankshaft, wobble surface, swash surface, cam surface, or other known piston actuator.
Due to the number of cycles that the piston undergoes during operation, piston failure, and thus compressor failure, is often attributable to wear experienced by the piston components. The wear experienced in a system with a solid reciprocating piston is particularly significant, as the weight of the piston increases stress on joints and the entire conditioning system. The greater the weight of the components, especially of the piston, the greater the risk of mechanical failure, for example, at welds or other points susceptible to fatigue and friction. Thus, overall piston life is not as great as is often desired. A further disadvantage of solid pistons is that they require additional work to actuate, reducing the overall efficiency of the compressor and air conditioning unit. The reduced efficiency and amount of material required add to the cost of solid pistons.
In another compressor design, hollow pistons are used as an alternative to solid pistons. However, known hollow pistons are not well accepted, particularly in the context of motor vehicle air conditioner compressors. In one respect, this is because known hollow pistons are formed from separate pieces of material. Creating a hollow piston from separate pieces of material increases the complexity and cost of manufacturing the piston, creates waste through scrap material created by circular cutouts, and increases susceptibility to wear, stress, and damage (e.g., at the weld site where the inner and outer portions are attached to the middle surface of the piston). Friction, for example, may cause pieces that have been joined together to break apart from one another. Accordingly, there is a need for a lower cost piston assembly that has good stability and performance relative to known solid and hollow pistons and that can be implemented in current or future compressor designs.